Etching methods are used in many fields of technology, such as in semiconductor technology, to remove unwanted material. For this purpose it is advantageous to have several suitable etchants for a particular material, in order to flexibly meet requirements concerning the etch rate, the accuracy and reproducibility of the etch rate, the handling (safety) as well as requirements relating to other materials that are also exposed to etchants. If small quantities of material are to be removed, it is desirable for etching to proceed slowly, so that the process as such can be readily controlled. For the removal of great quantities of material, on the other hand, it is preferable to have a high etch rate, to curtail the time required for etching. In particular for the removal of small quantities of material the etch process must be readily reproducible, since it is difficult to follow. To this end it is necessary for the composition of the etchant to be constant and for etching to be performed under constant conditions. In many instances the etch process is directed to generating an etched pattern, i.e., only predetermined areas of the workpiece to be processed may be attacked by the etchant. In these cases, the areas of the workpiece which are not to be attacked are covered with what is known as an "etch mask". For this purpose it is essential that during the etching of the non-masked areas the mask is not destroyed, thus becoming useless. This means, in other words, that the etchant employed etches selectively the material to be etched, so that the mask material is either attacked not at all or noticeably less than the material to be etched. Semiconductor technology is a field in which selective etching plays an important role.
In many instances it is not desirable for the selectivity of an etchant to be such that the material to be etched is heavily attacked, whereas the surrounding materials are not at all or only negligibly affected. In semiconductor technology, materials containing Si, such as Si.sub.3 N.sub.4, SiO.sub.2 and Si, are frequently used side-by-side. It may be advantageous that in cases where Si.sub.3 N.sub.4 or SiO.sub.2 are to be etched selectively, SiO.sub.2 and Si and Si.sub.3 N.sub.4 and Si, respectively, are also attacked, even though only to a limited degree. If, for example, an Si.sub.3 N.sub.4 layer covered with an SiO.sub.2 mask is to be etched, it may well happen that the SiO.sub.2 mask is defective, so that parts of the Si.sub.3 N.sub.4 areas to be etched are covered with SiO.sub.2 residues. If in such cases the Si.sub.3 N.sub.4 is etched by an etchant that only attacks Si.sub.3 N.sub.4, it is necessary to either remove the SiO.sub.2 residues in an additional step preceding the actual Si.sub.3 N.sub.4 etch process or to put up with an imperfectly etched Si.sub.3 N.sub.4 layer. If, on the other hand, an etchant is used which, in addition to Si.sub.3 N.sub.4, attacks SiO.sub.2, although on a reduced scale, the SiO.sub.2 residues and the Si.sub.3 N.sub.4 not covered with the non-defective etch mask can be removed in one etching step.
Etchants for materials containing Si are known. Thus, for example, the etchants best known for SiO.sub.2 are hydrofluoric acid (HF) and hydrofluoric acid buffered with ammonium fluoride (NH.sub.4 F), respectively, which attack Si and Si.sub.3 N.sub.4 hardly at all. Buffered hydrofluoric acid is favorable for high etch rates (&gt;500 A/min.). For low etch rates, on the other hand, it is necessary to use a buffered hydrofluoric acid containing a great quantity of NH.sub.4 F and a small quantity of HF. As NH.sub.4 F is hydrolysed according to the equation: 2NH.sub.4 F + H.sub.2 O = NH.sub.4 F . HF + NH.sub.4 OH, the hydrolysis is a function of the temperature and equilibrium is obtained slowly, the percentage of the existent small quantity of HF is considerably reduced and uncontrollably changed by the generated NH.sub.4 OH. Therefore, low etch rates are difficult to reproduce. Etchants primarily used for Si are those containing HF and an oxidation agent, preferably nitric acid (HNO.sub.3). By these etchants SiO.sub.2 is etched at least equally well as Si. Difficulties may be encountered, because in the usual Si etchants Si.sub.3 N.sub.4 is etched either not at all or essentially less than Si.
The etchant most frequently used for Si.sub.3 N.sub.4 is hydrous phosphoric acid. A method used to this end is described, for example, in German Pat. No. 1,614,909. The most concentrated phosphoric acid used for this method contains 15 percent water. With this or higher percentages of water neither SiO.sub.2 nor Si is noticeably attacked. In addition to the temperature, the etch rate of Si.sub.3 N.sub.4 in phosphoric acid is essentially influenced by the water concentration, and since phosphoric acid has to be heated for etching, it is either necessary in the interest of a constant etch rate to quantatively condense all of the evaporated water by complicated precautionary measures and to refeed it into the etch tank, or to put up with Si.sub.3 N.sub.4 etch rates that become gradually lower and lower.
In the Journal of the Electrochemical Society, Vol. 114, p. 869 ff. (1967) van Gelder and Hauser published an article "The Etching of Silicon Nitride in Phosphoric Acid with Silicon Dioxide as a Mask". The authors found that in a highly concentrated, aqueous solution containing over 85 percent phosphoric acid the etch rate of Si.sub.3 N.sub.4 increases as the water content increases and that the etch rate of SiO.sub.2 increases as the water content decreases. From the authors' test results it can be concluded that in 100 percent, i.e., anhydrous, phosphoric acid SiO.sub.2 is etched more intensely than Si.sub.3 N.sub.4. Gelder and Hauser, too, were confronted with the alternative of either etching in an open beaker, putting up with a steadily increasing SiO.sub.2 and a steadily decreasing Si.sub.3 N.sub.4 etch rate, or to condense the evaporating water by complicated precautionary measures and to steadily and continuously refeed it into the etching solution.
Van Gelder and Hauser in their above-mentioned article refer to etch rates of SiO.sub.2 and Si.sub.3 N.sub.4 in an H.sub.3 PO.sub.4 /P.sub.2 O.sub.5 melt (indicated P.sub.2 O.sub.5 content: &gt;10.4%), as shown by the two curves at the right hand side of FIG. 4 of the specification. This data does not indicate that Si.sub.3 N.sub.4 could be peferentially etched with respect to SiO.sub.2 at higher P.sub.2 O.sub.5 contents because the SiO.sub.2 etch rate appears to be rapidly increasing with increasing P.sub.2 O.sub.5 content.